The present invention relates to photosensing switching devices. More particularly, the present invention provides control of a lamp as a function of the ambient light level. The present invention is especially adapted for use with existing lighting fixtures.
Various switching control circuits have been proposed to provide automatic and unattended operation of outdoor and indoor lighting systems. Such circuits often employ a photosensing device to energize one or more lamps in response to ambient light levels, from dusk to dawn. A common problem with such switching control circuits is that the photosensing device often will sense the controlled lamp as ambient lighting, producing a flickering of the lamp. The switching control circuit switches the controlled lamp on in response to sensing a diminished ambient light level and then switches the controlled lamp off in response to sensing light produced by the controlled lamp, which is mistaken for the ambient light level. The prior art has attempted to resolve this flickering problem with sophisticated circuitry including delay circuits and multiple photosensors.
Another problem with prior art switch control circuits is that those employing thyristors or triacs, which control lamp operation by adjusting the phase angle of the gating signal. Such phase controlled circuits produce a substantial voltage drop across the thyristor which significantly reduces the voltage available for lighting and waste power.